Digital devices for measuring the frequency of an electrical signal are known, all of which operate basically in the manner that a counter counts pulses introduced by an oscillator during a time period determined by successive passages through zero of an electrical signal the frequency of which is to be measured. The counter reading at the end of the time interval then represents a measurement of the frequency to be measured.
By way of example, reference may be had in this connection to the published German application 31 49 165 A1 in which a circuit arrangement for measuring the frequency of an alternating voltage is described. Upon the commencement of each period of the alternating voltage, a time is started having a duration which corresponds to the shortest expected period of the alternating voltage. After the execution of this time, a following counter is started into which pulses of an oscillator are introduced until the end of the period being monitored is indicated by a control pulse. The frequency is calculated from the reading of the counter.